At the present time, the materials used for acoustic absorption are mainly materials with a porous matrix such as so-called porous materials (polyurethane foam, etc.) or so-called fibrous materials (glass wool, palm fibre, etc.). It is easy to integrate these materials into acoustic panels. In addition, the panel thus obtained is lightweight and has good acoustic attenuation in a major part of the frequencies of the audible spectrum.
However, these materials do not afford good attenuation in very low frequency sounds, that is to say for frequencies of around 50 Hz to 1000 Hz with thin panels with a thickness of around 5 to 10 cm, corresponding for example to the noise emitted by an engine ticking over. This is particularly true for frequencies where the corresponding wavelength is greater than four times the thickness of the material.
To overcome this problem, the solution commonly adopted consists of increasing the thickness and mass of the porous matrix by combining layers of different porous materials. The main drawback lies in greater size and mass of the acoustic panel.
Studies, in particular that of Groby et al. “Enhancing the absorption coefficient of a backed rigid frame porous layer by embedding circular periodic inclusions” (JASA, 130(6): 3771, 2011), have shown that the use of resonators such as split rings or Helmholtz resonators arranged in a layer of porous material made it possible to significantly absorb the low-frequency sounds incident on such a structure.
These structures thus significantly increase acoustic absorption. The physical phenomena have been revealed in several scientific publications, such as the article by Allard and Atalla “Propagation of sound in porous media: modelling sound absorbing materials” (Chapter 5, page 85, Wiley, 2009) with regard to the acoustic behaviour of a porous material, and in the scientific article by Groby et al. cited above with regard to the behaviour of the resonators included in the porous matrix.
Thus these structures make it possible to attenuate the acoustic energy through viscous and thermal losses. The resonators integrated in the porous matrix act as diffusers, reflecting the incident acoustic wave in all directions. Some of the acoustic energy is also absorbed because of the resonance of the resonators at their resonant frequency that depends on the dimensional characteristics of the resonator.
However, at the present time, though the efficacy of such a cell had been demonstrated, no particular industrially applicable geometry has yet been proposed. This is because the aforementioned studies were limited to demonstrating the advantage of a porous-matrix cell integrating a resonator. In addition, though the coefficient of absorption with such a cell is greater over the entire range of low frequencies up to 6000 Hz, it is greater than 0.8 only for frequencies above 2500 Hz and is less than 0.5 for very low frequencies below 1700 Hz.
In the scientific publications “Absorption of a rigid frame porous layer with periodic inclusions backed by a periodic grating”, JASA, 129(5), May 2011, and “Enhancing the absorption coefficient of a backed rigid frame porous layer by embedding circular periodic inclusions”, JASA, 130(6), December 2011, Groby et al. propose a numerical model comprising a layer of porous material comprising infinitely rigid cylinders, the arrangement of which makes it possible to form a diffraction grating. The cylinders used in the numerical model are cylinders defined numerically as infinitely rigid so that they cannot be assimilated to acoustic resonators.